The other problem is that TSB didn't go live by stages, but all at once. Has TSB decided to go live on a subset of customers, it would have had the possibility of seeing the storm that was brewing and scale back and correct the issues before the whole thing blew up in their faces.
In any case, they certainly got a big bang, just not the one they were hoping for.
Oh, and a note to all high-level managers : let this be a lesson to you. Never believe someone who tells you that there will be no migration issues to worry about and we can do it all in one go.
It never happens like that.